The present invention relates generally to an apparatus for generating electrical current from the nuclear decay process of a radioactive material. In a specific, non-limiting example, the invention relates to an energy cell (e.g., a battery) for generating electrical current derived from particle emissions occurring within a confined volume of radioactive material (e.g., tritium gas).
Radioactive materials randomly emit charged particles from their atomic nuclei. Examples are alpha particles (i.e., 4He nuclei) and beta particles (i.e., either electrons or positrons). This decay process alters the total atomic mass of the parent nucleus, and produces a daughter nucleus, having a reduced mass, that may also be unstable and continue to decay. In such a nuclear decay series, a fraction of the original material is consumed as energy, and eventually, a stable nucleus is formed as a result of successive particle emissions.
The principal use of controlled nuclear decay processes relates to generation of energy producing heat sources. Two of the best-known examples are nuclear reactors for producing electric power, and radioisotope thermal generators (RTGs) used in connection with various terrestrial and space applications.
Nuclear reactors have a heat-generating core that contains a controlled radioactive decay series. Heat generated within the core during the decay series is transferred to an associated working fluid, for example, water. The introduction of heat into the working fluid creates a vapor, which is in turn used to power turbines connected to electric generators. The resulting electricity is then wired to a distribution grid for transmission to users.
RTGs are also heat-generating devices, wherein electricity is produced by one or more thermocouples. The principle of operation of a thermocouple is the Seebeck effect, wherein an electromotive force is generated when the junctions of two dissimilar materials, typically metals, are held at different temperatures. RTGs are typically used for space applications due to their reasonably high power-to-weight ratio, few (if any) moving parts, and structural durability. RTGs also supply power in space applications where solar panels are incapable of providing sufficient electricity, for example, deep space missions beyond the orbit of Mars.
Previously, a major drawback when attempting to use energy derived from a nuclear decay series to power devices in remote locations has been an inefficiency of the energy conversion process. For example, it has proven difficult to achieve much greater than a ten percent energy conversion rate, especially when the energy is transferred via a thermodynamic cycle as described above.
As seen in prior art FIG. 1, a schematic representation of an energy generation process achieved by emission of a charged particle from the nucleus 1 of a radioactive material 2 is shown. Provided that an electric field is maintained between positive electrode 3 and negative electrode 4 by a potential difference 5, a charged decay particle creates electron/hole pairs that migrate toward naturally attractive electrodes 3 and 4. If a resistive load Ω completes the circuit such that positive charges 6 and negative charges 7 recombine, power is generated by the induced current flow.
Electrical current directly derived from a nuclear decay process is frequently referred to as an “alpha-voltaic” or “beta-voltaic” effect, depending on whether the charged particle emitted by a particular nucleus is an alpha particle or a beta particle, respectively.
A description of efforts to exploit the nuclear decay process of a radioactive material is found in A Nuclear Microbattery for MEMS Devices, published by James Blanchard et al. of the University of Wisconsin-Madison in August, 2001, and incorporated herein by reference. Blanchard et al. sought to develop a micro-battery suitable for powering a variety of microelectromechanical systems (“MEMS”). Advantages of using such devices to power MEMS include a remote deployment capability, high power-density as compared to other conventional micro-energy sources, and long-term structural durability.
Other references to nuclear batteries include U.S. Pat. No. 6,479,920 to Lal et al.; U.S. Pat. No. 6,118,204 to Brown; U.S. Pat. No. 5,859,484 to Mannik et al.; and U.S. Pat. No. 5,606,213 to Kherani et al of which are incorporated herein by reference. None of these nuclear batteries have been developed commercially for practical applications.